Staking, which is a kind of press forming, has been a well known technology as a mechanical juncture means to join metal materials. “Staking” is a generic word to represent to join more than two members, one of which member is pressed to make shaping for such juncture. For the purpose to obtain enough mechanical strength of the join, it is important that the shaping portion of the shaping material by staking is deformed by staking or pressing within a limit of the plastic deformation of the shaping material and the shaping portion is tightly inlaid in the joined material.
Define of some words used in the description of the present invention are hereby provided. They are “shaping material” as a shaping material by staking, “joined material” as a material which has a open hole to which the shaping portion (as defined as in the following) of the shaping material is inlaid and which is joined with the other material or materials, and “shaping portion” as an exserted bump portion wherein the bump is formed in the shaping material and the rest of the bump is deformed to be inlaid in the open hole.
To deform the shaping portion within the limit of the plastic deformation of the shaping material, the technology called as “electric staking” is well known such that the exserted portion formed in the shaping material is softened by the heat given by the thermo-generation in the electric resistance of the shaping portion through which the electric current flows and the shaping portion is deformed to the shape of the final inlay to fit into the open hole of the joined material by pressing the staking die against the shaping portion.
FIG. 4A, FIG. 4B, FIG. 4C and FIG. 4D show the conventional electric staking process. FIG. 4A is the status of a shaping material, a joined material and a staking die. FIG. 4B shows a deformation process and FIG. 4C shows a post status after a staking has been done in the staking process. FIG. 4D shows a perspective view of the shaping portion 15c′ of the shaping material 5.
As shown in FIG. 4A, the conventional staking die 1′ has a cylindrical form and the pressing surface of the die is flat. Therefore, in the process of the staking especially when the pressing surface of the electric staking die 1′ presses the shaping portion 15b′ simultaneously the electricity is applied to the staking die, a small gap is made between the pressing surface and the shaping portion 15b′. An electric discharge 16 tends to be generated in this gap. The corner of the shaping portion is melted by the discharge heating and needle-like thorns 17 are made.
As shown in FIG. 4B, the shaping portion 15b′ is heated by the electric current in the staking process using the electric staking die 1′ therefore is easy to be deformed. However the peripheral part of the shaping portion exposes to the ambient air and the temperature is lower than the center part of the shaping portion through which the much electric current relatively flows, therefore a large temperature slope is made such that the temperature of the peripheral portion is relatively low. While the shaping portion is kept pressed, the peripheral portion is hard to be deformed since the temperature is low and the limit of the plastic deformation is higher than the heated part of the shaping portion. This results in generating cracks in the peripheral area of the shaping portion.
When the needle-like thorns 17 are made on the surface of the shaping portion 15c′ after staking, the out look is poorly rough and additional works to remove the thorns 17 are needed, which wastes the manufacturing time.
The cracks 18 cause poor outlook as well as joint strength with the joined material becomes week. Moreover, since the cracks cannot be amended, the quality control has been done in a manner such that larger cracks than a standard allowance are regarded as failure staking products and the shaping material and the joined material must be abandoned. Therefore there is a risk of manufacturing problem such as large amount of the product failure due to generating such cracks.